Use of the absolute phase in frequency modulated continuous wave plasma reflectometry

Area of Science:

• Plasma physics
• Fusion energy research
• Wave propagation

Background:

• Frequency Modulated Continuous Wave (FMCW) reflectometry is crucial for measuring plasma density profiles in fusion devices.
• Traditionally, the absolute phase of the beat signal in FMCW reflectometry is neglected due to phase ambiguity (shifts ≥ 2π) between sweeps.
• This ambiguity limits the achievable spatial resolution in density profile measurements.

Purpose of the Study:

• To investigate the potential of utilizing the absolute phase of the beat signal in FMCW reflectometry.
• To resolve phase ambiguity under specific plasma conditions for enhanced spatial resolution.
• To apply this improved method for analyzing plasma phenomena like edge localized mode precursors.

Main Methods:

• Analysis of FMCW reflectometry signals, focusing on the absolute phase of the beat signal.
• Observation of "quiet plasma conditions" where phase shifts between sweeps are significantly less than 2π.
• Treating the reflectometer signal as an interferometer signal to leverage phase information.

Main Results:

• Demonstrated that under quiet plasma conditions, phase ambiguity can be resolved.
• Achieved substantial improvements in spatial resolution for plasma density profile measurements.
• Enabled the first quantitative measurement of plasma boundary displacement caused by edge localized mode precursors.

Conclusions:

• Utilizing the absolute phase in FMCW reflectometry significantly enhances spatial resolution.
• This technique provides a powerful new tool for diagnosing fusion plasmas, especially for dynamic events.
• The study also estimated errors associated with both absolute phase and conventional frequency measurements.